Apparatus of zone refining and controlling solute segregation in solidifying melts by electromagnetic means



Sept- 1963' .A.TlLl ER E L I 3,401,021

UTE

APPARATUS I OF Z REFINING AND O TROLL SOL SEGREG ATION IN SOLIDIFYINGLTS ELECTROMAGNETIC ME Original Filed Aug. 1, 1961 5 Sheets-Sheet lINVENTORS William C. Johnston 8 William A. Tiller.

ATTORN a W. A. TILLER ETAL Sept. 10, 1968 APPARATUS OF ZONE REFINING ANDCONTROLLING SOLUTE SEGREGATION IN SOLIDIFYING MELTS BY ELECTROMAGNETICMEANS 5 Sheets-Sheet 2 Original Filed Aug. 1, 1961 Sept. 10, 1968 w. A.TILLER ETAL 3,

APPARATUS OF ZONE REFINING AND CONTROLLING SOLUTE SEGREGATION INSOLIDIFYING MELTS BY ELECTROMAGNETIC MEANS Original Filed Aug. 1, 1961 5Sheets-Sheet s Sept. 10, 1968 w A TILLER ETAL 3,401,021

APPARATUS OF ZONE REFINING AND CONTROLLING SOLUTE SEGREGATION INSOLIDIFYING MEL-TS BY ELECTROMAGNETIC MEANS 5 Sheets-Sheet 4 OriginalFiled Aug. 1, 1961 Sept. 10, 1968 w. A. TILLER ETAL APPARATUS OF ZONEREFINING AND CONTROLLING SOLUTE SEGREGATION IN SOLIDIFYING MELTS BYELECTROMAGNETIC MEANS 5 Sheets-Sheet 5 Original Filed Aug. 1, 1961 A .mF

United States Patent 3,401,021 APPARATUS OF ZONE REFINING AND CONTROL-LING SOLUTE SEGREGATION IN SOLIDIFYING MELTS BY ELECTROMAGNETIC MEANSWilliam A. Tiller, Menlo Park, Calif., and William C. Johnston,Pittsburgh, Pa., assignors to Westinghouse Electric Corporation,Pittsburgh, Pa., a corporation of Pennsylvania Original application Aug.1, 1961, Ser. No. 128,446, now Patent No. 3,203,768. Divided and thisapplication Aug. 26, 1965, Ser. No. 482,733

5 Claims. (Cl. 23-273) ABSTRACT OF THE DISCLOSURE Apparatus forcontrolled solification of molten materials which comprises a series ofcoils, usually a plurality of single turn coils, disposed adjacent thesolid-liquid interface of the body of molten material being solidified,the coils being sequentially energized by alternating current to causemovement of the melt at such interface by a magnetic field varying inforce and direction.

This application is a division of our application Ser. No. 128,446,filed Aug. 1, 1961, and assigned to the present assignee, issued asPatent 3,203,768.

The present invention relates to apparatus for improved zone refiningand for controlling solute segregation in the solidification of afusible material by electromagnetic mixing and, in particular, toapparatus for electromagnetically stirring molten material as itfreezes.

It is well known in the prior art to zone refine metals, semiconductorsand the like, by disposing a long bar of metal, for example, within anelongated receptacle of a refractory material and passing the receptaclewith the metal slowly through a heating means such as an induction coilwhich melts only a small zone of the metal bar. As the receptacle moves,the molten zone traverses the bar of metal and carries impurities withit. In practice, a bar of the metal is moved through a refiningapparatus comprising a series of induction coils whereby progressivemolten zones are formed and traverse the bar of metal in a single passthrough the apparatus. For impurities whose solubility in the moltenphase exceeds that in the solid phase, the impurities are concentratedin the molten zone and are progressively swept toward the end of the barof metal. After the bar has been cooled, the end containing theimpurities may be cut off and the remainder is highly purified metal.The process may also be employed for growing a single crystal by seedingthe initial molten zone with a single crystal.

It is also well known in the prior art that during crystal growth from amelt comprising a major component with a minor solute componentdissolved therein, the solute concentration in the liquid immediatelyahead of the solid-liquid interface changes due to the partitioning ofsolute between the two phases. This solute distribution limits the speedat which homogeneous and relatively perfect crystals, single orpoly-crystals, can be grown. Control of this solute distributionprovides a measure of control over the state of aggregation of a solidmaterial prepared by the freezing process applied to a melt.

In purifying a material by zone refining, the efliciency of the processas to impurity refinement may be increased and the solute distributionmay be controlled by suitable stirring of the molten zone and minimizingthe thickness of the dilfusion boundary layer at the crystal-meltinterface. Also, to grow a crystal of contant composition or to attain aparticular solute distribution requires exact and constant control ofthe diffusion boundary layer during the freezing process. Naturalconvection in the liquid 3,401,021 Patented Sept. 10, 1968 "ice meltcannot'be relied upon for control of the diffusion boundary layer;therefore, a forced motion must be introduced into the liquid to gainthe required degree of control. p

' It is known to apply a constant magnetic field adjacent one end of amolten zone in a plane parallel to the solidliquid interface. Thistechnique is deficient in the following respects: (1) the induced fluidflow can be only in planes parallel to the interface since the directionof current flow is axialin the bar; and (2) the charge must comprise anelectrically conductive material. The latter limitation prevents thepurification of non-conductive materialsby this prior art stirringtechnique.

The object of the present invention is to provide an apparatus forremoving soluble and/ or insoluble impurities and control solutesegregation from a fusible solid having a molten zone by subjecting itto either zone melting or normal freezing, the apparatus comprising amovable and variable magnetic field means disposed adjacent the moltenzone.

Another object of the invention is to provide apparatus for controllingsolute segregation in a fusible material by electromagnetic stirringcomprising means for passing an alternating current to electromagneticcoils disposed about a molten portion of a body of the material incontact with a solidified portion of the material so that a force orforce couple is applied to said molten portion to effect any one of thefollowing liquid movements: (1) rotating the liquid in the planesperpendicular to the body axis,

(2) rotating the liquid in planes parallel to the body axis, (3)translating the liquid in planes parallel to the body axis, or (4)translating the liquid radially in planes perpendicular to the bodyaxis, or combinations of two or more, whereby a relatively high degreeof control is exercised over the configuration and magnitude of meltflow, the thickness of the diffusion boundary layer at the freezinginterface and the amount of solute entering the solid from the melt.

A futrher object of the invention is to provide means for producingeither certain uniform or non-uniform distributions of solute elementsin a fusible solid by creating either a molten zone in the body or bymelting the total charge and performing a normal freeze operation,wherein the volume of molten material is subjected to a torque bycreating a moving magnetic tfield within it by applying current directlyto coils of electromagnets disposed about the molten portion therebycontrolling the thickness of the diffusion boundary layer at thefreezing interface.

Other objects of the invention will, in part, be obvious and will, inpart, appear hereinafter.

In order to more fully understand the teachings of the invention,reference should be had to the following detailed description anddrawings, of which:

FIGURE 1 is a perspective view of an electromagnetic device designed torotate the liquid in planes perpendicular to the specimen axis;

FIGURE 2 is a perspective view of an electromagnetic device designed torotate the liquid in planes parallel to the specimen axis;

FIGURE 3 is a perspective view of an electromagnetic device designed totranslate the liquid in planes parallel to the specimen axis;

FIGURE 3A is a perspective view of the electromagnetic coils;

FIGURE 4 is a perspective view of an electromagnetic device designed totranslate the liquid radially in planes perpendicular to the specimenaxis;

FIGURE 4A is a plan view of the electromagnetic coils of FIGURE 4;

FIGURE 5 is a perspective view of another electromag- 3 netic devicedesigned to rotate the liquid in planes perpendicular to the specimenaxis;

FIGURE 6A is a photomicrograph of a cross-section of material solidifiedwithout stirring magnetically; and,

FIGURE 6B is a photomicrograph of a cross-section of a similar materialprocessed in accordance with the teachings of the invention.

In.accordance with the present invention and in attainment of theforegoing objects, there is provided an I variable magnetic field, themagnetic means being so associated and shaped as to fit closely to atleast a portion of a body of molten material so that a high proportionof the magnetic field penetrates into the molten material and causes itto be stirred in varying directions and with varying force, particularlyat the solid-liquid interface.

For the removal of impurities, a molten portion is created in a part orin all of the body of material by means of a heating coil or othersource of heat. Freezing is caused to progress into the molten portion,there being at least some solid-liquid interface having a diffusionboundary layer adjacent to it. The volume of molten material issubjected to a force or force-couple by the penetration of the movingmagnetic field into the volume by the applied alternating currentcooperating with the coils of the electromagnets disposed about themolten portion. The molten material, which may be either a metal, asemiconductor, or even a non-conductor, may thereby be stirred with avariety of fluid flow configurations such as rotation in a planeperpendicular to the solid-liquid interface, or in the plane of thesolid-liquid interface or translation radially or longitudinally in themolten portion depending on the arrangement and form of theelectromagnets.

Convection in the molten liquid reduces the thickness of the diffusionlayer at the freezing solid-liquid interface and improves the effectivepartition coefiicient. Convection also lessens the possibility ofconstitutional supercooling and dendrite formation. Therefore, themoving magnetic thickness of the diffusion boundary layer and controlsthe concentration and distribution of the minor constituents orimpurities entering the sold phase. During stirring, the molten materalis progressively frozen to a solid thereby causing the newly frozensolid phase to be substantially altered in impurity concentration(reduced for partition coefficients less than unity and increased forpartition coefficients greater than unity). These magnetic means alsomay be employed to produce a specified distribution of a particularconstituent or impurity in the body.

The electromagnetic stirring devices may also be employed in growingsingle crystals or polycrystalline materials by maintaining the entirecharge in the molten state, applying the magnetic stirring system andprogressively pulling a solid crystal from the melt while stirring.

Both the degree of purification during zone refining and the particularsolute distribution frozen into a crystal during crystal growth, dependupon the effective partition coefiicients, k, of the solute underconsideration. The effective partition coeflifiicient, k, depends uponthe normalized freezing velocity, vB /D, where v is the freezingvelocity, 6 is the diffusion boundary layer thickness and D is thesolute diffusion coeififiicient in the liquid. The minimum tme requiredto produce a certain degree of purification during zone refining occurswhen va /D approximately equals 1. Thus by reducing 6 by electromagneticstirring, the freezing velocity, 1 can be increased maintaining va /Dequal to 1 and the cost of producing this purified material decreases asv increases. For producing a certain solute distribution one mustmaintain v6 /D=constant and and again the time and cost of thisoperation can be reduced as v is increased by reducing 6 throughelectromagnetic mixing while maintaining v6 /D=constant. One can thussee the advantages produced by controlling the diffusion boundary layerthickness, 5 The diffusion boundary layer thickness, 5 is alteredbyliuid moving in a loop consisting of components parallel andperpendicular to the solid-liquid interface. This particular fluid flowconfiguration may be considered as either primary circulation orsecondary circulation depending upon whether this fluid flow is produceddirectly by applied forces or as a by-product of a different primaryflow. For example, when a magnetic field rotates in planes parallel tothe solid-liquid interface it produces a fluid rotation in these planesas a primary circulation; however, the diffusion boundary layerthickness, (t is altered not by this primary circulation but by asecondary circulation pattern. The electromagnetic devices of thisinvention therefore are important as a consequence of either theirprimary or secondary circulation patterns.

The velocity, u, of the fluid flow determines how much 6 is altered. Thevelocity, u, depends upon the liquid viscosity, 1/ and the applied forceor force-couple, F. Control of 5 therefore, may be produced by variationof F. The force or forcecouple in the outer surface of the liquid due toan electromagnetic field increases linearly with (l) the liquidconductivity, 0, (2) the square of the magnetic field strength, H and(3) the frequency of the applied magnetic field, Thus, for a givenmaterial F may be increased and thus 6 decreased by increasing either 7or H.

The applied force, F, decreases with distance into the liquid and fallsto l/e, where e is the base for natural logarithms, of its surface valuein a skin depth of thickness, L, where L is inversely proportional to 0fThus, for a given material, if f is very large, the applied force isconcentrated in the surface layers of the liquid whereas when f is smallthe force may be applied throughout the total volume of liquid. Further,if it is necessary to penetrate a conducting crucible to stir theliquid, a high value of f is prohibitive.

In any particular stirring application the configuration of both theliquid and the solid-liquid interface determine the best electromagneticfield configuration. The dimensions of the melt and the presence orabsence of a conducting crucible will determine the best field frequencyto use. The conductivity, viscosity and diffusion boundary layerthickness desired will determine the best magnetic field strength touse. For non-conducting liquids, the introduction of conducting rotorswill allow the liquid to be stirred by the action of the electromagneticfield.

Referring to FIGURE 1, an electromagnetic device 10 is applied to thestirring of the liquid zone 11 in a zone refining apparatus 2. Theapparatus 2 comprises a sealed tube 3 mounted on rollers 4 forreciprocation inside of a heating coil 5 which surrounds a hollowinsulating tube 6 of a refractory such as silica or alumina. A boat ofgraphite or the like carries an elongated specimen 9 which is heated bythe coil 5 to produce a narrow molten zone 11. A magnetic field isapplied to the molten zone 11 in the region of the freezing interface bycrossed magnetic yokes 13 and 14 of a ferromagnetic material such asnickel iron, one of which produces a magnetic field at molten zone 11 inplanes perpendicular to the axis of the elongated specimen 9 and theother produces a magnetic field in planes orthogonal to both thespecimen axis and the other magnetic field. A rotating magnetic field isinduced by passing two-phase current to the field coils 15 and 16 of theyokes 13 and 14 from a suitable source such as an alternating currentgenerator (not shown). This field configuration rotates the liquid inplanes perpendicular to the specimen axis and the control 6 by secondarycirculation if the freezing interface is perpendicular'to thespecimenaxis and primary circulation if the'freezirig interface isparallel to the specimen axis.

The coils and 16 require fairly high voltages to produce an effectivestirring current, because of the resistance of the yokes 13 and 14. Forinstance with a two inch pole face separation, to produce a field of 300gauss in the molten zone requires a voltage of from 400 to 600 voltsacross the coils and capacitors are required in the circuits to balancethe reactive current. The line voltage is of the order of 50 to 100volts. e n

- The arrangement of FIGURE 1 can be modified to dispose-three sets 'of'magnetic yokes'and coils supplied with three phase current so astoproduce -a rotating magnetic field in the plane perpendicular to thespecimen axis. I

An electrically more efiicient and lower voltage arrangement forstirring is illustrated in FIGURE 2.

Referring to FIGURE 2, a heating coil 17 is adapted to produce a moltenzone in a zone melting apparatus 18. A rotating magnetic field isapplied to the liquid melt zone in the region of the freezing interfaceby applying two-phase alternating current to field coils 19 and 20,which do not contain any ferromagnetic core materials associatedtherewith. In FIG. 2, coil 19 is shown out of position, and in operationwould be moved into contact with heating coil 17. Coils 19 and 20produce a magnetic field in planes parallel to the specimen axis. Fieldcoils 21 and 22 which are disposed to produce a magnetic field in planesperpendicular to the specimen axis have small focusing pole pieces 23and'24 wherebythis latter field is applied to the melt more efiiciently.The magnetic field configuration from the joint operation of coils 19,20, 21 and 22 rotates the liquid melt in planes parallel to the specimenaxis and controls 5,, by primary circulation when the freezing interfaceis perpendicular to the specimen axis and, in part, by secondarycirculation when the freezing interface is parallel to the specimenaxis.

Good results have been obtained with the apparatus of FIGURE 2 appliedto apparatus similar to that of FIGURE 1, with a line voltage of 10volts being adequate to produce the same degree of stirring as with theFIG. 1 arrangement. Consequently, the FIG. 2 apparatus is far moreefiicient, and needs less critical electrical insulation and capacitorsthan does the FIG. 1 apparatus.

Referring to FIG. 3, an electromagnetic device 52 comprising threestacked circular field coils 54, 56 and 58 and their respective leadsfrom a three-phase current source is applied to elongated barsolidification apparatus 50 suitable for growing long crystals oringots. The apparatus 50 comprises a quenching tube 60 containing aquenching fluid 62, either a gas or a liquid, at the lower end of aheating tube 64 in which an elongated graphite cylinder 66 containing ametal or semiconductor or the like, is heated to a temperature to meltthe contents of the cylinder 66. A suitable means such as a wire 68connected to a driving means (not shown) lowers the graphite cylinder ata given speed into the quenching fluid 62 so that the melt solidifies ata predetermined rate from the botton end. The electromagnetic device islocated at the solid-liquid interface area in the cylinder 66.

FIGURE 3A shows the details of single twin loop field coils 54, 56 and58, FIG. 3, their connectors 53 and 55 (as well as one not shown) andindicates the direction of current flow in the coils.

The magnetic field is applied to the liquid melt in the region of thefreezing interface by applying a three-phase current to loop field coils54, 56 and 58 which produces a longitudinally translating magnetic fieldparallel to the specimen axis. This field configuration rotates theliquid primarily in planes parallel to the specimen axis and controls5,, by primary circulation when the freezing interface is eitherperpendicular to or parallel to the specimen axis.

Referring to FIGURES 4 and 4A, a pancake electromagnetic device 70 isillustrated. A moving magnetic field is produced by applying athree-phase alternating current to a pancake coil 69 comprising a seriesof three field coils 71, '72 and 73, each of which comprises twoseparated single turns as shown, which produce a radially translatingmagnetic field. The field coils are electrically connected to anelectrically conductive metal base member 74 by means of invertedL-shaped supports 76 and mounting screws and metal straps 79 and areelectrically insulated from the supports by means of an insulating plate78. Alternating current from a threephase source' is applied to each ofsupports 76 which conduct the current to coils 71, 72 and 73.

If a cylindrical melt 81 is placed on the upper surface of the pancakecoil 69 so that it is perpendicular to the axis'of the-melt, the primaryflow of melt will be in planes perpendicular to the melt axis. Further,if one or more of 'such pancake coils 69 are placed adjacent to andparallel to the cylindrical wall of a crucible containing a melt or ametal weld puddle, the primary-flow of liquid is in planes parallel tothe specimen axis. In either case, this coil structure produces a strongsecondary flow along the axis of the pancake coil.

-While='the coils are arranged in three sets for threephase currentoperation, .it should be understood that any number of phases may beobtained by increasing the sets of concentric coils.

Also, in lieu of the arrangement of coils in FIGURE 4, a series ofgeometrically similar conductors may be employed, for example, eitherstraight or curved parallel bars may be electrically connected insuitable sequence to a polyphase current source so as to produce aneffective traveling magnetic field passing from bar to bar. By placing aflat crucible with a melt on such parallel bar array, translationalstirring will proceed from one side to the other as the electricalcurrent magnetic field passes from bar to bar.

Referring to FIGURE 5, a cylindrical electromagnetic device isillustrated. There are no ferromagnetic core materials in this form ofapparatus and therefore low power is required to secure good stirring. Amoving magnetic field is produced by applying a three-phase alternatingcurrent to a cylindrical array of field coils which produce a rotatingmagnetic field in planes perpendicular to the specimen axis. Three pairsof vertical coils 92-92, 9696 and 100-100 connected by upper leads 94,98 and 102 are arranged in a cylindrical arrangement in ring insulators104 and 106. An elongated cylindrical crucible containing a melt isintroduced into the space within the ring insulators and permitted tosolidify either radially from the exterior surface or longitudinally.The general coil arrangement of FIG. 5 may be employed with higherpolyphase currents whereby more parallel coil bars than the six shown,may be arranged in a cylindrical array. Melts of liquids contained incrucibles of varying crosssectional configurations may be placed withinan array of parallel conductors spaced about the irregular or warpedsurfaces so as to stir the melt as it progressively solidifies into ahomogeneous solid. By this means, rectangular, elliptical andstreamlined shaped bars and rods may be produced. The rotating magneticfield effects primary fluid movement of the liquid phase in planesperpendicular to axis of the cylindrical crucible.

Since ferromagnetic core materials are not present, induction heatingcoils can be disposed within conductors of the device withoutdetrimental coupling. Thus zone refining can be readily carried out inthe apparatus of FIG. 5.

The following examples are illustrative of the teachings of theinvention:

EXAMPLE I posed in an open topped graphite boat was placed in a 30millimeter diameter five foot long length silica tube mounted forlongitudinal movement on rollers. The zone refining apparatus wassimilar to that shown in FIGURE 1. The tube was closed at each end bybrass caps. Both caps contained a stainless steel needle valve throughwhich the system was evacuated and the central portion of the tube andthe graphite boat was heated to a temperature of 250 C. Then argon wasadmitted into the tube and the valves were closed. The power for themagnetic stirring to coils 15 and 16 of the apparatus was supplied at100 volts by a 400 c.p.s. two-phase generator. The alloy charge was zonerefined by the action of induction coil 4 and the alternating currentmagnetic stirring of the magnets. After a single pass was made on thecharge, the bar was cut from end to end into a plurality of cylindricalsamples and the concentration of tin in each sample was determined. Itwas determined through calculation that the diffusion boundary layerthickness at the freezing interface was exponentially dependent upon thesquare of the magnetic field strength for a 400 c.p.s. field rotating inthe plane of the interface.

In this way, va /D was reduced by a factor of 50 with a field strengthof 350 oersteds which, in turn, was produced by a power input of about300 watts.

However, when the electromagnetic device configuration illustrated inFIGURE 2 was used to melt a similar lead-tin alloy specimen, va /D wasreduced by a factor of about 200 at the same field strength which wasproduced by a power input of about 100 watts to the stirring magnets.The lower value of 5 arose in the second case because a primarycirculation was relied upon rather than secondary circulation. The lowerpower input was present in the second case because there were no ironlosses.

EXAMPLE II A bar of thermoelectric material consisting of 20% bismuthselenide and 80% bismuth telluride was solidi fied from a melt using acrystal growth apparatus with out any magnetic stirring apparatus.Another bar of the same composition was solidified employing a magneticstirring system employing the apparatus of FIG. 3. After examiningsamples taken from each charge, the results showed that the diffusionboundary layer at the freezing interface was reduced in thickness by afactor of 20 with the magnetically stirred charge as compared to thecharge processed without stirring.

With reference to FIGURES 6A and 613, there is shown an inhomogeneousand a homogeneous sample respectively, the latter produced in accordancewith this Example II. FIG. 6A reveals that severe microsegregation ofthe chemical constituents on a size scale of about 200 microns hasoccurred during the freezing process of an unstirred melt, whereas FIG.6B reveals that, for the magnetically stirred liquid, relatively littlemicrosegregation occurred during the freezing process. Themicrosegregation in the solid arises because of a particular solidliquid interface morphology that develops under conditions ofconstitutional supercooling in the liquid adjacent to the interface. Asthe diffusion boundary layer thickness is reduced, the degree ofconstitutional supercooling is reduced and the probability of obtainingthe type of solid-liquid interface morphology that leads tomicrosegregation in the solid is reduced.

The consequences of this type of microsegregation in the solid forthermoelectric materials is that the efiiciency is reduced, for example,the bar illustrated in FIG. 6A will have definitely poorerthermoelectric properties than that illustrated in FIG. 6B.

EXAMPLE III An elongated cylindrical charge of mercury was placed in thezone refining apparatus of FIGURE 1. Several steel balls were disposedin the mercury. When the magnetic fields were applied to the mercury thesteel balls move violently in the mercury. By freezing the mercury witha refrigerant, and heating only a narrow zone, the arrangement enablesstirring of the mercury with great efficiency.

Other materials such as garnet, sulfur, selenium, refractory silicatesand other inorganic electrical insulators may also be magneticallystirred by this method of this Example III. Also, saline water insolidified form may be stirred in order to remove the salt by zonepurification.

The steel balls in all these instances maybe replaced by nickel balls orboron carbide cylinders, or electrically conductive, chemicallynon-reactive material.

It should be understood that it is possible when employing the devicesdescribed herein to (1) increase the multiplicity of phases of currents,(2) increase the multiplicity of poles, (3) increase or decrease thevelocity of the traveling waves produced by these devices and (4) modifythe shape of the individual poles. For example, the pancake coilillustrated in FIGURE 4 may consist of a parallel array of straightconductors such as shown in FIG. 5, to produce primary fluid flow of amelt in a direction parallel to the surface of the conductors.

Furthermore, the electromagnetic stirring devices are delineated hereinas being particularly applicable to relatively small zone refiningapparatus. However, the stirring devices may be employed in refiningrelatively large metal ingots up to inches or even feet in diameter andmany feet in length.

It is intended that the above description and drawings be construed asillustrative and not in limitation of the invention.

We claim as our invention:

1. In a controlled solidification apparatus, an elongated receptaclewith a melt of material therein, means for cooling one end of thereceptacle to cause progressive solidification of the melt from said oneend, a plurality of magnetic single turn looped field coils coaxiallyarranged with respect to each other and the receptacle, the magneticcoils being capable of concentrating a high proportion of the magneticenergy thereof in the melt at the solid-liquid interface thereof, meansfor moving the receptacle along said axis with respect to the pluralityof said magnetic single-turn looped field coils, means for connectingthe coils to a source of polyphase alternating current, energizingadjacent coils in a predetermined sequence whereby a translatingmagnetic field is produced in the melt to electromagnetically stir themolten zone as it freezes at the solid-liquid interface.

2. In apparatus for obtaining controlled segregation and distribution ofsolutes, heating means for progressively melting a suitably containedbar of material, means for causing the heating means and the bar ofmaterial to move with respect to each other, cooling means forsolidifying the molten material, a plurality of stacked magneticsingle-term looped field coils circumscribing the bar of material, thecoils being disposed closely adjacent to each other and beingelectrically connected in predetermined sequence and being capable ofconcentrating a high proportion of the magnetic energy thereof at thesolid-liquid freezing interface of the molten zone to translate theliquid in planes perpendicular to the interface, and circuit means forconnecting the said magnetic single-turn looped field coils to a sourceof polyphase alternating current in predetermined sequence whereby themagnetic field applied by said coils to the molten zone can continuallyvary in force and in direction.

3. In a controlled solidification apparatus, a plurality of single turnmagnetic looped field coils concentrically disposed with respect to eachother and forming a surface, the magnetic coils being connected inpredetermined sequence to a source of polyphase alternating currentwhereby'upon passage of the alternating current thereto, the coils willproduce a radially translating magnetic field, the said magnetic coilsbeing applied to a receptacle of a molten material wherein thereceptacle has a surface substantially parallel to said coil surface, toconcentrate a high proportion of the magnetic energy thereof in theliquid melt to translate the liquid radially in planes parallel to thesurface of the coils and circuit means for connecting the magnetic coilsto a source of alternating current whereby the magnetic field applied bysaid coils to the molten zone can continually vary in force and indirection.

4. For use in a controlled solidification apparatus, a plurality ofgeometrically similar substantially parallel single turn loop fieldconductors, electrically insulating means holding the conductors insubstantial parallel arrangement, means electrically connecting theconductors in suitable sequence to a polyphase alternating currentsource so as to produce an efiective traveling magnetic field passingfrom conductor to conductor in regular sequence, there beingsubstantially no ferro-magnetic material associated with the conductors,the magnetic energy of the parallel conductor arrangement being appliedto a liquid melt to translate the melt in direction of travel of themagnetic field in planes parallel to the arrangement of the conductors.

5. In a controlled solidification apparatus, a plurality ofgeometrically similar single-turn looped field conductors, electricalmeans attached to and holding the conductors in a substantially parallelcylindrical arrangement, means electrically connecting the conductors insuitable sequence to a polyphase alternating current source so aS toproduce an effective traveling magnetic field passing from conductor toconductor in regular sequence around the cylindrical arrangement tocircumscribe a body of molten material and applying the magnetic energythereof to the liquid material to move the liquid in direction parallelto movement of the magnetic field.

References Cited UNITED STATES PATENTS 2,826,666 3/1958 Cater 23-2732,890,940 6/1959 Pfann 23301 3,203,768 8/1965 Tiller et a1. 23273 OTHERREFERENCES Zone Melting, by William G. Pfann, 1958, John Wiley and Sons,Inc., Chapman and Hall, Limited, pp. 80-92.

NORMAN YUDKOFF, Primary Examiner.

G. P. HINES, Assistant Examiner.

